Particular
attention was placed in 2016 on determining the colour phases of the
juvenile Honey-buzzard, as shown in Table 28:

Total
juveniles seen

Phase
unknown

Phase
known

Total

dark

brown

rufous/ruddy

pale

barred

purple

Total

Study
Area bred

82

15

44

10

5

5

1

2

67

%
phase-known total in study area

66

15

7

7

1

3

N
England migrants

26

1

15

0

7

3

0

0

25

%
phase-known total in migrants

60

0

28

12

0

0

Overall

108

16

59

10

12

8

1

2

92

%
phase-known total overall

64

11

13

9

1

2

Table
28. Incidence of colour phases in juvenile Honey-buzzard in 2016 bred
in the Study Area in SW Northumberland and in migrants passing
through northern England

Colour
phases were noted in the field followed by checks made on photographs
obtained. The colour was assessed on the coverts of the underwing and
on the underbody; the head colour was also noted, particularly on
paler individuals. The difference between dark- and brown-phase birds
is subjective to some extent with the shade on a cline from almost
black to a neutral brown; the distinction was often made when
studying a pair of juveniles where one looked significantly darker
than the other. In the study area in SW Northumberland, 82 juveniles
were studied in post-fledging situations from 5/8-25/9, including one
juvenile just outside the current study area; the colour phase for 15
of these birds (18%) could not be determined as the birds were too
distant or the lighting was too poor such as the bird being in
silhouette. Presumed migrants were also studied with 26 juveniles
being assessed from 9/9-4/11; the colour phase could not be
determined for 1 of these birds (4%). These presumed migrants
included 22 in Northumberland over the whole quoted period and 4 in
the Lake District from 9/10-12/10. These presumed migrants are
considered to be almost entirely Scottish-bred, moving slowly through
northern England. For the overall figures,108 juvenile were studied
of which 16 could not be assessed (15%).

The
results in the study area show that 66% were dark-phase, with 15%
brown, 7% rufous/ruddy, 7% pale, 3% purple and 1% barred. There is
therefore a wide range of plumage but 81% are of the dark/brown
types, indicating that to a casual observer, juvenile Honey-buzzard
appear sombre colour-wise. The smaller sample of migrants shows a
broadly similar picture except for a significantly higher proportion
in the rufous/ruddy category (28%). Adding the figures together to
give an overall picture gives 64% dark and 11% brown, making 75% in
the sombre class. Minor but still significant phases are 13%
rufous/ruddy and 9% pale, with just 2% purple and 1% barred.

Worth
quoting from Ferguson-Lees,
James, and Christie, David A, Raptors of the World, Christopher Helm
(2001):

p.341.
Western Honey Buzzard is then monotypic but, in parallel with its
eastern counterpart, polymorphic to extent of being arguably more
diversified in colour and pattern (chiefly head, underbody and
wing-linings) than almost any other wild bird species. Often lumped
simply as 'typical', 'pale' and 'dark', about ten adult and seven
morphs are distinguishable -- some common, some much scarcer -- with
further intergrades; adults can be classed as 'white-bellied' (male
only), 'densely barred', 'thinly barred', 'black-blotched',
'black-spotted', 'sparsely-spotted', and the more unicolorous
'rufous, 'olive-brown', 'dark brown' and 'melanistic'; counterparts
of the last four are also found in juvenile plumages, along with
'buff', 'light' and 'whitish'.

This
quote shows the variability and complexity of the juvenile plumage.
My classification focuses on what can be seen at some distance in the
field. Dick Forsman, in Flight Identification of Raptors of Europe,
North Africa and the Middle East, Christopher
Helm (2016), notes:

p.56-57.
Although juveniles come in many different plumage-types, uniformly
dark brown birds predominate and account for about 90% of all
juveniles. The remaining birds are lighter below, from sandy buff to
pure white, variably streaked or mottled, often with a pale or
whitish head, with showy dark 'sunglasses' in many. The underwing
coverts vary in accordance with the general plumage colour and
pattern … The upperparts also vary according to the general
colouration of the plumage …

On
my classification the pale birds make up 9%, in line with the
estimate by Forsman of 10%. The sombre birds make up 75%, less than
the 90% estimated by Forsman, but some of my darker ruddy-plumaged
birds and the purple-phase birds would I feel be considered as simply
dark by Forsman so our estimates seem to be in reasonable agreement.
Forsman does show a ruddy-tinged dark juvenile in picture 27 (p.61).